Since March 2010, when U.S. President Barack Obama signed the Affordable Care Act into law, the ACA has been at the center of American politics. Tea Party activists and their allies in the Republican Party have tried to stymie the law at nearly every turn. The Republican-controlled House of Representatives has voted more than 40 times in favor of repealing or defunding it, and last October the House allowed a partial shutdown of the federal government in an attempt to block or delay the law. The controversy surrounding the ACA shows no sign of ending anytime soon.

Obamacare, as the law is commonly known, is the most significant reform of the U.S. health-care system in half a century. It aims to increase the share of Americans who have health insurance, improve the quality of health insurance plans, and slow the growth of health-care spending. But the fight over the law is about more than just health-care policy, and the bitterness of the conflict is driven by more than just partisan polarization. Obamacare has become the central battleground in an ongoing war between liberals and conservatives over the size and scope of the U.S. government, a fight whose origins stretch back to the Great Depression and the New Deal.

Opponents of President Franklin Roosevelt’s innovations were silenced when the New Deal’s reforms were locked in during the Truman and Eisenhower years, and the U.S. welfare state took another leap forward under Lyndon Johnson, whose Great Society agenda expanded public help for the poor and created the government-administered health insurance programs Medicare and Medicaid. But the following decades saw few major additions and some notable setbacks, including the failure of President Bill Clinton’s health-care reform effort in 1994.

The passage of Obamacare has caused such controversy in part because it seems to signal a new stage of government activism, leading some conservatives to oppose it as a decisive and possibly inexorable turn to the left. “Precisely because the Affordable